Multi-Telescope Studies of Neutron Stars

Abstract

Neutron stars are unique stellar remnants with extreme properties, as their density and magnetic field. Their study can be the key to a number of unanswered problems in fundamental physics and astronomy, ranging from stellar evolution to strong field gravity. One of the best ways of studying these objects is with observations at radio wavelengths, the efficiency of which can be vastly improved with the combination of data from multiple radiotelescopes. In this thesis, we use the largest European radiotelescopes for performing high quality studies of the properties of objects belonging into two separate categories of neutron stars, millisecond pulsars and magnetars. In the first part of this thesis, a complete description of the observing systems and calibration procedures for the multiple telescopes used is presented. Specifically, all observations were made with the European Pulsar Timing Array (EPTA) telescopes, which are the Effelsberg 100m radiotelescope in Germany, the Lovell 76m radiotelescope in UK, the Westerbork 94m equivalent synthesis radiotelescope in the Netherlands and the Nan\c cay 94m equivalent decimetric radiotelescope in France. In addition, the different procedures for the data acquisition and improvement of the latest and archival data of the Effelsberg radiotelescope are described. Finally, the techniques and advantages of the combination of multi-telescope data sets are being presented. In the second part of the thesis we concentrate on the study of millisecond pulsars using the pulsar timing technique. Specifically, we analyse a set of 15 millisecond pulsars from the Effelsberg source list, showing that most of them are good candidate sources for the EPTA efforts to detect gravitational waves in the nano-Hertz regime. We present, in most cases only for the Effelsberg data set, improved preliminary results for their astrometric, spin and binary parameters. Finally, we report on the complete timing analysis of one of these sources. Specifically, we present results from the high precision timing analysis of the pulsar-white dwarf binary PSR J1012+5307 using 15 years of EPTA multi-telescope data. All the timing parameters have been improved from the previously published values, at least by an order of magnitude. In addition, a parallax measurement is obtained for the first time for PSR J1012+5307, being consistent with previous optical estimations from the WD companion. Combining improved 3D velocity information and models for the Galactic potential the complete evolutionary Galactic path of the system is obtained. While a new intrinsic eccentricity upper limit is acquired, being one of the smallest calculated for a binary system and providing evidence for the stellar evolution of this system, a measurement of the variation of the projected semi-major is also constraining the systems orbital orientation for the first time. Finally, combining the fact that PSR J1012+5307 is an ideal laboratory for testing alternative theories of gravity, with a measurement for the first time of the change of the orbital period of the system, stringent, general, theory independent upper limits for the dipole gravitational wave emission and the variation of the gravitational constant are being derived. In the final part of this thesis, we study the category of magnetars and specifically the case of the first radio emitting anomalous X-ray pulsar (AXP) J1810-197. With a simultaneous and quasi-simultaneous multi-frequency and multi-telescope campaign from July 2006 until July 2007 we obtained flux density measurements and spectral features of this 5.5-sec radio-emitting magnetar. We monitored the spectral evolution of its pulse shape which consists of a main pulse (MP) and an interpulse (IP). We present the flux density spectrum of the average profile and of the separate pulse components. We observe a decrease of the flux density by a factor of 10 within 8 months and follow the disappearance of one of the two main components. Although the spectrum is generally flat, we observe large fluctuations of the spectral index with time. We conclude that AXP J1810-197 is not like any other radio pulsar we know with spectral properties and temporal fluctuations differing remarkably from normal pulsars. Significant variability exists on all considered time scales, from pulse to pulse, day-to-day and over the time of weeks and months. Analysis of the interstellar scintillation for AXP J1810-197 shows that only some of the variability is affected by scintillations and most of it is due to intrinsic variations, better described by a model of turbulent magnetosphere. Further analysis on the single pulse properties of AXP J1810-197 confirms these results.